GB2062684A

GB2062684A - Cast Steel Tools

Info

Publication number: GB2062684A
Application number: GB7938617A
Authority: GB
Original assignee: Gepipari Technologiai Intezet
Current assignee: Gepipari Technologiai Intezet
Priority date: 1979-11-07
Filing date: 1979-11-07
Publication date: 1981-05-28
Also published as: GB2062684B

Abstract

In a casting process for the production of steel cutting tools, one or more of a rare earth metal, zirconium and niobium, preferably cerium, is added in an amount of Re 0.1-0.8% to a low-carbon steel melt, the melt is cast and then the surface layer of the cast is enriched to a minimum of 0.6% carbon content, to a depth of at least 1 mm. The addition may also take place during pouring. The steel used contained: <IMAGE>

Description

SPECIFICATION A Method of Manufacturing Cast Alloy Steel Cutting Tools This invention relates to a method of manufacturing cutting tools from cast alloy steel. Many cutting tools are made of high-speed steel. There are two main methods of manufacturing them.

According to one method, the steel is cast into a block or ingot and the crude steel thus obtained is then extended by a hot-forming method, such as forging and rolling, to obtain an intermediate product the dimensions of which correspond to those o' the desired tool. This preliminary forming serves not only to obtain an intermediate product -- for its dimensions -- ready to be further processed but also to disintegrate or eliminate during this process the crude primary carbides, especially the carbide eutectics, that formed in the course of the solidification of a steel containing usually more than 0.7% carbon and also carbide-forming alloy elements (Cr, Mo, W, V). As far as the quality of the final product is concerned, the size the primary carbides are disintegrated to is of decisive importance.

That is why the distribution and the lattice structure of the carbides is prescribed both in international and national standards, and the presence of crude carbide particles is prohibited in the quality test provisions for tool steel.

Another method for the utilisation of high-speed steel in the manufacture of cutting tools consists in casting the tool immediately to its final shape. A disadvantage of this method consists in the difficulty of eliminating the said rough carbide particles from a product obtained in this way. In the layer next to the surface, however, the carbide-forming process is somehow different, the carbide particles are less crude or large and the working capability of cast high-speed tools can often equal that of tools made by rolling or forging of intermediates. There are countries where cast cutting tools are specially widely utilised.Their main advantage lies in that their manufacture is less expensive since a lot of steps can be avoided in the course of obtaining the tool in its final shape from the crude steel, considering that the shape of the casting differs from that of the tool only by a small grinding allowance.

On the other hand, the basic disadvantage of the cast high-speed steel lies in that the carbide texture is remarkably inhomogeneous, consisting mainly of crude particles so that the toughness of the tool is insufficient and it is also less resistant against the very severe abrasive effect caused during the cutting process.

It is an aim of the invention to eliminate the said disadvantages of the cast high-speed steel tools according to the prior art and to obtain cast cutting tools containing the desired amount and composition of alloy components usual with high-speed steels while having improved properties such as increased toughness and wear resistance as well as versatility of the cutting edge so that the said properties are equal to or even better than those of rolled or forged high-speed cutting tools. This aim may be attained by eliminating the crude carbide texture and creating a fine carbide texture of suitable composition in the surface layer of the tool.The invention is based upon the following conception: It is undoubtedly an advantage of the casting method that the final shape (but for a small grinding allowance) can be obtained in a single step but this very fact also has a disadvantage, namely that their is no way of controlling the homogeneity and fineness of the carbide texture in the course of the said single step.If the one-step shaping is maintained but performed in a low-carbon state of the steel, and subsequently a further step such as carburisation is performed in order to increase the carbon content of the surface layer up to the desired amount (equal to that in the prior art), the carbon content of the working layer will not differ from that of the steel prepared according to the prior art whereas the fineness of the carbide texture as well as its homogeneity could be remarkably improved and thus also the life of the cutting edge and the working capacity of the tool, and even the toughness of the tool, wouid increase.After all, the method according to the invention constitutes an optimum compromise between the minimum need for labour typical of the casting method and the improved texture as obtainable in a very labour-intensive way in the forging process, since the need for labour is only slightly increased -- not commensurably with the need for labour, and especially the need for skilled labour required by the forging method - and the texture of the working layer is nevertheless, equal to and the toughness even better than those obtained by the forging method.

According to the invention one or more rare earth metal components, such as cerium -- totalling up to 0.10.8% (all percentages throughout are by weight) - are added to the molten steel at the end of the melting process in order to improve the texture of the melt, the carbon content of the said steel being at the end of the pouring process set to 0.20.4%, and after having cast the tool from the said melt, the carbon content of the surface layer is enriched up to at least 0.6%, preferably 0.7%, by diffusion in order to obtain a fine carbide texture.

As a result of the above method, the amount of the alloy components in the cutting tools according to the invention is basically equal to the standardized composition of the high-speed steels except that the said rare earth addition is also provided for. Recent experiments have shown that the quality improvement according to the invention can also be obtained if only the main novel features, i.e. the casting in low-carbon state, the use of some additive, and the subsequent carburisation of the surface layer are maintained whereas the said additive may - wholly or partially - be other than a rare earth metal, e.g. zirconium or niobium, the said additive totalling also in this case up to 0.10.8%.

The composition of the alloy may therefore be the following: 2-6% chromium, 0-24% tungsten, 0-10% molybdenum, 0-10% cobalt, 0.58.0% vanadium, and 0--39/0 titanium.

The steel contains also small amounts of the usual impurities such as manganese, silicon, phosphor and sulphur.

One of the features of the novel method consists in that the carbon content of the steel at the end of the pouring process is relatively small: 0.20.4%. The setting of this value is performed according to prior art.

Another feature consists in the adding of 0.10.8%, preferably 0.20.3%, of one or more rare earth metal component(s) and/or zirconium and/or niobium, the addition taking place at the end of the melting process or during the casting. This addition is performed in order to obtain a fine texture. The addition may be "misch-metal", ferrocerium, silicon-zirconium orferro-niobium, respectively.

A preferred feature consists in the subsequent carburization of the surface layer of the cast tool (to a depth not less than 1 mm) by a diffusion method to obtain a carbon content of at least 0.6%, preferably at least 0.7%. The diffusion can be performed by any known method used in carburization.

The said methods being well known, a more particular description of same can be obviated while remarking that any known method of the thermo-chemical heat treatments can be applied: solid state carburizing such as case-hardening, carburizing in a gaseous medium, vacuum-carburizing included, carburizing in a liquid such as a salt bath.

The carburization is to be performed upon the castings having already approximately the final shape (but for a small grinding allowance) the said shape being obtained either immediately by an accurate casting or by a subsequent cutting. When the carburization has been performed, a heattreatment usual with high-speed steels can be applied, such as annealing and/or a final heat-treatment consisting of hardening and tempering, as is usual with high-speed steels. The above method leads to a satisfactory texture in the tool, i.e. the carbon content of the inner part (core) remains low and no rough carbide particles are present, therefore the core is tough while the surface layer comprises fine carbide particles as the consequence of the diffusion, i.e. a texture which can be obtained by no other technology except powder metallurgy which latter is however, much more expensive.It is well known that the limits concerning the chemical composition of the high-speed steels are determined by the crudeness of the carbide particles in the melt and the difficulty of disintegrating same. The said limits can, however, be extended by applying the above-described novel method. If the surface layer is only subsequently carburized, there is no obstacle in the way of increasing the amount of carbide-forming alloy components (V, Mo, W, Ti) deliberately since the subsequently diffused carbon results in fine carbide particles causing considerably fewer difficulties than any carbon added already during the melting process.

The casting cutting tools prepared according to the invention are far less expensive than the cutting tools obtained by forging or rolling, respectively, yet their capability and essential features are on a par with the latter.

The experiments performed with the tools prepared according to the invention are specified hereinafter, purely by way of example: 1. Carburizing in a Solid Medium a) the solid carburizing agent: SZ 15/5+5% Na acetate Carburizing temperature: 1 2000C Carburizing time: 6 hours Thickness of the carburized layer: 2 mm (0.6% C-content) Original C-content of the steel: 0.32%.

b) The solid carburizing agent: SZ 15/5+5% Na acetate +5% CaCO3 + 10 cm3 water Carburizing temperature: 1 2000C Carburizing time: 6 hours Thickness of the carburized layer: 3.0 mm (0.6% C-content) Cutting performance increase: 58%.

2. Carburizing in Gaseous Medium a) Gaseous carburizing medium: PB-gas+O2 (Ratio: 1:3) Carburizing temperature: 9400C Carburizing time: 2 hours Thickness of the carburized layer: 1.5 mm (0.6% C-content) Original C-content of the steel: 0.32% b) Gaseous carburizing medium: PB-gas+02 (Ratio: 1:1) Carburizing temperature: 9400C Carburizing time: 4 hours Thickness of the carburized layer: 1.1 mm (0.6% C-content) Cutting performance increase: 8-10%.

Claims

1. A method of manufacturing cast cutting tools for use in high speed cutting, using steel containing 26% chromium, 0-24% tungsten, 0-10% molybdenum, 0.5-8.0% vanadium, 010% cobalt, 0-3% titanium, and the usual impurities such as manganese, silicon, phosphor, and sulphur wherein there is added to the steel at the end of the melting process or during the pouring an addition consisting of one or more rare earth metals in a total amount between 0.10.8%, preferably 0.2-0.3%, the carbon content of the steel at the beginning of the pouring process being set to 0.20.4%, and the surface layer of the tool cast from the thus obtained steel is subsequently carburized by diffusion to obtain a carbon content of not less than 0.6%, preferably not less than 0.7%.

2. A method of manufacturing cast cutting tools for use in high-speed cutting, using steel containing 2-6% chromium, 0-24% tungsten, 0-10% molybdenum, 0.5-8.0% vanadium, 010% cobalt, 0-3% titanium, and the usual impurities such as manganese, silicon, phosphor and sulphur, wherein there is added to the steel at the end of the melting process or during the pouring an addition consisting of one or more rare earth metals and/or zirconium and/or niobium in a total amount between 0.10.8%, preferably 0.2-0.3%, the carbon content of the steel at the beginning of the pouring process being set to 0.20.4%, and the surface layer of the tool cast from the thus obtained steel is subsequently carburized by diffusion to obtain a carbon content of not less than 0.6% preferably not less than 0.7%.

3. A method as claimed in claim 1 or 2, wherein the rare earth metal(s) is/are added in the form of misch-metal.

4. A method as claimed in claim 1 or 2, wherein the rare earth metal is added in the form of ferrocerium.

5. A method as claimed in any of ciaims 1 to 4, wherein the surface layer is carburised to a depth of not less than 1 mm.

6. A method as claimed in any preceding claim, substantially as herein particularly described.

7. A cast alloy steel or a cutting tool made therefrom whenever made by the method claimed in any preceding claim.